The present invention generally relates to cardiac pacemakers and other types of implantable medical devices that can be programmed and/or analyzed following implantation using an external diagnostic/programmer system. Particularly, the invention relates to a high-speed digital telemetry system that includes a transmitter and a corresponding external receiver, with the transmitter achieving high data transmission rate without significantly increasing the design complexity.
Implantable devices are implanted in a human or animal for the purpose of performing a desired function. This function may be purely observational or experimental in nature, such as monitoring certain body functions; or it may be therapeutic or regulatory in nature, such as providing critical electrical stimulation pulses to certain body tissue, nerves or organs for the purpose of causing a desired response. Implanted medical devices such as pacemakers, perform both observational and regulatory functions, i.e., they monitor the heart to ensure it beats at appropriate intervals; and if not, they cause an electrical stimulation pulse to be delivered to the heart in an attempt to force the heart to beat at an appropriate rate.
In order for an implantable device to perform its functions at minimum inconvenience and risk to the person or animal within whom it is used, some sort of noninvasive telemetry means must be provided that allows data and commands to be easily exchanged between the implantable device and an external device. Such an external device, also known as a controller, programmer, or monitor, provides a convenient mechanism through which the operation of the implantable device can be controlled and monitored, and through which data sensed or detected by the implanted device can be transferred out of the implanted device to an external (non-implanted) location where it can be read, interpreted, or otherwise used in a constructive manner.
As the sophistication and complexity of implanted devices has increased in recent years, the amount of data that must be transferred between an implanted device and its accompanying external device or programmer, has also increased. This, in turn, has resulted in a search for ways to effectuate such a data transfer at high speed. Understandably, an implanted device, due to its limited physical dimensions, allows for limited complexity of the electronic circuitry it can incorporate. Thus, in order for an implantable telemetry system to meet its design goals, it must transfer data at a high speed while preserving the relative simplicity of the circuitry.
An exemplary conventional telemetry system with a data transfer speed of 8 kbps (kilobits-per-second), more exactly 8192 bps, is described in U.S. Pat. No. 4,944,299 to Silvian. That telemetry system uses a carrier frequency of 8192 Hz, a frequency selected to be higher than a computer monitor vertical sweep generator fundamental and its harmonics, and below the horizontal sweep generator fundamental. This choice is based on the presumption that the computer monitor represents the primary source of interference for the carrier frequency. This 8 kbps telemetry system uses 1 bit per symbol and further uses a combined Amplitude Modulation (AM) and Phase Shift Keyed (PSK) modulation to limit the necessary bandwidth.
While the design of such a system is advantageous, there still remains a need for a telemetry system that allows high data rate transfer of information, and that does not substantially increase the design complexity of the implantable device.
The present invention addresses the foregoing need by providing an improved telemetry system. According to a preferred embodiment, an implanted unit (IU) of the telemetry system allows a high-speed transfer of digital data with minimal complexity of the electronic circuitry. A corresponding external unit is capable of decoding the high-data-rate transmitted information and, in turn, communicates with the implanted unit using, for example, pulse amplitude modulation (PAM).
In the preferred embodiment, the implanted unit-to-external device data rate is 32768 bps (32 k), a four-fold increase over conventional data transmission rates, without increasing the carrier frequency. In particular, the 32 k data is transmitted by the implanted unit, using a unique implementation of the QAM (quadrature amplitude modulation) method. Whereas traditional implementations of the QAM scheme require complicated components such as sinewave carriers, multipliers and filters, the present invention generates the required symbols from squarewave signals that are readily available as digital signals.
In particular, the simulated sinewaves are generated within the transmitter by an inverting amplifier stage with variable input resistance determined by a pair of switches that are ultimately controlled by 16 k and 32 k clocks in the implanted unit. Data is, in turn, encoded by changing the amplitude and polarity of the simulated sinewaves. Quadrupling of the data rate is achieved, not by decreasing the symbol period or increasing the carrier frequency, but rather by taking advantage of the orthogonality of I and Q components, whose phases are in quadrature. By implementing a two-bit modulation capability within each of the two transmitters, four bits of information (Ibit1, Ibit0, Qbit1, Qbit0) are encoded during one symbol period of 122 xcexcs (equal to 1/8192 Hz) are encoded in an I transmitter signal while Qbit1 and Qbit0 are encoded in the output signal of the transmitter Q, 90 degrees out of phase with the signals from the I transmitter. These signals are fed into a telemetry coil for transmission to the external device.
The I transmitter has two digital inputs, Ibit1 and Ibit0, each with a pair of possible values, a logic 0 or a logic 1. Corresponding values Qbit1, Qbit0 are encoded in the output of the Q transmitter. The effects are the same within the individual transmitters, and thus, the digital inputs may be referred to generically as Bit1 and Bit0. In a preferred embodiment, Bit1 determines the polarity of the simulated sinewave during the symbol period. Bit0 independently controls the amplitude of the output, resulting in an absolute peak (normalized) amplitude of 1 or 0.5. Therefore, it may be understood that during the symbol period the resulting, modulated, simulated sinewave produced by an individual transmitter may take on any one of four possible functional forms: sin (t+xcfx86), xe2x88x92sin (t+xcfx86), 0.5 sin (t+xcfx86), or xe2x88x920.5 sin (t+xcfx86), depending on the logic values of Bit1 and Bit0, where xe2x80x9csinxe2x80x9d is understood to be a symbolic representation of the simulated sinewave synthesized from squarewaves, and xcfx86 represents the relative phase of the synthesized sinewaves. Phase quadrature between the signals produced by the two transmitters allows the embedded information to be distinguished and decoded by the external device.
In an exemplary implementation, the I transmitter and Q transmitter are identical. Phase quadrature, as required by the QAM method of the present invention, is achieved by including an inverter and a D flip-flop within the integrated circuit but external to the Q transmitter. Both the I and Q carrier frequencies are 8192 Hz and are, thus, equal to the conventional carrier/symbol rate. Higher frequency clocks of 8 k (8192 Hz), 16 k (16384 Hz), and 32 k (32768 Hz) that are required for this implementation, are generated from a 32 k crystal oscillator and by a chain of dividers.
Another important feature of the present invention is the novel implementation of symbol delimiters (start, end). By shifting both the I and Q symbol starting points 45 degrees with respect to the 8 k clock, the noise due to harmonics, generated by squarewaves, can be reduced. Thus, at both the symbol start and stop the worst-case amplitude jump is reduced and, in fact, is the same for both I and Q components.
In order to implement the foregoing design change on a conventional QAM scheme, a full amplitude, I transmitter-only signal is sent during a brief portion of the signal period. This allows a phase lock loop (PLL) in the external device to lock onto the exact phase of the I signal and thus discriminate its information from that of the Q signal. The full amplitude signal also allows the automatic gain control (AGC) within the external device to be simplified in its implementation, since it is able to lock onto a signal with no interference, with a larger magnitude and a larger signal-to-noise ratio.
In one embodiment, the external device (programmer)-to-IU data transfer rate will remain 8 kbps and will employ pulse amplitude modulation (PAM). However, to be fully compatible with the present implanted unit, a digital signal processing telemetry module (DTM) that forms part of the external device, can be modified to process QAM signals.